The present invention relates generally to audio sound systems and more specifically to audio sound systems which can decode from two-channel stereo into multi-channel sound, commonly referred to as "surround" sound.
Since Peter Scheiber's U.S. Pat. No. 3,632,886 issued in the 1960s, many patents have been issued regarding multidimensional sound systems. These systems are commonly known as 4-2-4 matrix systems, where four discrete audio signals are encoded into a two channel stereo signal. This encoded stereo signal can then be played through a decoder, which extracts the four encoded signals and feeds them to their intended speaker locations.
4-2-4 matrix designs were originally applied to the quadraphonic sound systems of the 1970s, but in recent years have become enormously popular for cinematic applications and, even more recently, home theater applications. Following the demise of quadraphonic sound, companies such as Dolby Laboratories adapted the matrix scheme to cinematic applications in an attempt to provide additional realism to feature films. The aforementioned Scheiber patent, as well as his subsequent patents U.S. Pat. Nos. 3,746,792 and 3,959,590, are the patents cited by Dolby Laboratories for the Dolby Surround.TM. system. Popular surround systems for cinematic and home theater applications typically provide discrete audio signals to four speaker locations--front left, front right, front center and rear surround. The rear surround environment is typically configured with at least two speakers, located to the left and right, which are each fed the mono surround signal.
Subsequent patents on 4-2-4 matrix systems have attempted to improve on the performance of the matrix. For example, the original passive systems were only capable of 3 dB of separation between adjacent channels (i.e. left-center, center-right, right-surround and surround-left), therefore it was desirable to develop a steered system which incorporated gain control and steering logic to enhance the perceived separation between channels.
Many prior art surround systems have utilized a variable matrix for decoding a given signal into multi-channel outputs. Such a system is disclosed in U.S. Pat. No. 4,799,260, assigned to Dolby Laboratories, as well as in U.S. Pat. No. 5,172,415 from Fosgate. Each of these patents disclose a variable output matrix which provides the final outputs for the system. Other designs, such as that shown in U.S. Pat. No. 4,589,129 from David Blackmer, disclose a system which does not include a variable output matrix but instead includes individual steering blocks for left, center, right and surround.
The evolution of the surround sound system has seen the developers of such systems progressively attempt to develop the technology which would allow audio engineers the ability to place specific sounds at any desired location in the 360.degree. soundfield surrounding the listener. A recent result of this can be seen with the development of Dolby Laboratories' AC3 system, which provides five discreet channels of audio. However, there are at least two major drawbacks to such a system: (1) it is not backward-compatible with all existing material, and, (2) it requires digital data storage--not allowing for analog recording of data (i.e. audio tape, video tape, etc.). A Dolby AC3-encoded digital soundtrack can not be played back through a Dolby Pro Logic system.
The inventions described in my U.S. Pat. Nos. 5,319,713 and 5,333,201 are major improvements over what has become commercially known and available as Dolby Surround.TM. and Dolby Pro Logic.TM., primarily in that those patents cited describe a means of providing directional information to the rear channels--a feature which the Dolby systems do not provide. This feature is very desirable in exclusive audio applications, as well as in applications where audio is synched to video (A/V), and is fully described in the above-cited patents. However, although the inventions described in my above-cited patents greatly improve on the previous designs, none of the matrix-based systems disclosed to date have provided a means of achieving independent left and right rear channels when decoded.
My currently pending U.S. patent application Ser. No. 08/426,055 discloses a means of providing additional discrete signals through the practice of embedding one or more signaling tones at the upper edge of the audio spectrum during the encode process. These tones can then be detected during the decode process to re-configure the system such that front left, center and front right channels become disabled--thus allowing for signals panned left, center and right to be fed exclusively to the rear left, overhead and rear right locations, respectively. The detection of an additional signaling tone can then reset the system configuration, if desired. Although this system provides a means of producing additional channels and is an improvement to existing systems, it does introduce drawbacks. For example, the practice of embedding tones within the audio spectrum introduces the possibility of them becoming audible to the listener, which is unacceptable. In addition, such a system could only be applicable to a limited number of recording mediums, due to the inherent limitations of mediums such as cassette tape and the optical soundtrack for 35 mm film.
It is desirable, therefore, to be able to encode five discrete audio signals down to a two-channel stereo recording and then have the ability to place specific sounds at any one of 5 or more predetermined locations as individual, independent sound sources when decoded--thus producing a 5-2-5 matrix system. A typical implementation of such a system might provide signals to left front, right front, center, left rear, and right rear speaker locations. There are numerous other embodiments of the invention with many other possible channel configurations, as will be apparent to those skilled in the art. It is, therefore, a primary object of the present invention to provide a matrix system which would decode a stereo signal into at least five stand-alone, independent channels. It is also an object of the present invention to achieve a matrix system which is compatible with all existing stereo material. Another object of this invention is to provide a matrix system which is compatible with material encoded for use with other existing surround systems. Yet another object of this invention is to provide a matrix system such that material specifically encoded for this system can be played back through any other existing decoding systems without producing undesirable results.